• On Earth, carbon circulates from one reservoir to another at various speeds and in various ways, certain compartments constituting more or less important reserves, more or less durable, or more or less accessible (Figure 5.1).

• High economic growth scenarios tend to assume high levels of progress in the efficiency of end-use technologies; however, carbon intensity reductions were found to be largely independent of economic growth assumptions.

• For example, for the most important anthropogenic greenhouse gas, carbon cycle models indicate that stabilisation of atmospheric CO2 concentrations at 450, 650 or 1,000 ppm would require global anthropogenic CO2 emissions to drop below 1990 levels, within a few decades, about a century, or about two centuries, respectively, and continue to decrease steadily thereafter.